1. Field of the Invention
The present invention relates to stretchable interconnects providing electrical connection in which conductive films or stripes are formed on or embedded within deformable substrates and the substrates may be pre-stretched before fabrication of the conductive film or stripe.
2. Related Art
A number of electronic circuits require low resistance connections between parts that are mechanically separate and/or can move against each other. Examples include: large-area electronics that can be bent or 3-D deformed; printed wire boards with creases along which they can be folded to achieve high density; and integrated circuits that move against their packages under the influence of thermal expansion. Typically, when such movement occurs the electrical contacts between interconnects and circuits are subjected to mechanical stress. If this stress results in mechanical debonding, the circuit also fails electrically.
Photolithographically patterned stretchable interconnects for electrically connecting electronic devices which are supported for movement relative to one another have been described. U.S. Patent Application No. 2002-0294701 describes a stretchable interconnect formed of a coiled conductor. The coiled conductor is formed by photolithography. In this technique, a negative or positive resist (photoresist) is coated onto an exposed surface of a material. The resist is irradiated in a predetermined pattern, and irradiated (positive resist) or nonirradiated (negative resist) portions of the resist are removed from the surface to produce a predetermined resist pattern on the surface. This can be followed by one or more procedures such as etching, plating, and the like. The coiled conductor is formed of a metal or alloy having a stress gradient extending through the thickness of the conductor. The interconnects become stretchable when a supporting substrate is removed from the interconnect.
Formation of ordered structures in thin films of metals supported on an elastomeric polymer have been described in Bowden, N. et al., Nature, 393, 146 (1998). The ordered structures were spontaneously generated by buckling of thin metal films owing to thermal contraction of an underlying substrate. Films from the vapor phase are deposited on a thermally expanded polymer of polydimethyl siloxane (PDMS). Subsequent cooling of the polymer creates compressive stress in the metal film that is relieved by buckling with a uniform wavelength of 20-50 micrometers. The waves can be controlled and oriented by relief structures in the surface of the polymer to provide intricate ordered patterns. It is described that the patterning process may find applications in optical devices.
Inherent flexibility of thin-film electronics can be used in a variety of applications. One approach to making flexible and deformable structures is to use polymer substrates. The flexibility of the polymer substrate offers application opportunities that utilize curved and/or deformable surfaces. Retina-shaped photosensor arrays described in Hsu, P. et al., Appl. Phys. Lett. 81, 1723-5 (2002), electro-active polymer actuators described in Pelrine, R. et al., Smart Structures and Materials 2001: Electroactive Polymer Actuators and Devices, edited by Y. Bar-Cohen, SPIE Proc. 4329, Bellingham, WA, (2001) pp. 334-349, or stretchable sensitive skin described in Lumelsky, V. J. et al. IEEE Sensors journal 1, 41 (2001) are electronic systems that combine electronic functions with the flexibility of plastic substrates.
Dielectric elastomer actuators with smart metallic electrodes made of silver were described in Benslimane et al., Smart Structures and Materials 2002, Electroactive Polymer Actuators and Devices, edited by Y. Bar-Cohen, 150 Proceedings of SPIE Vol. 4695 (2002). An elastomer film is spin coat on a mold for forming a corrugated quasisinusoidal profile. Thin metal films are deposited on the corrugated surfaces of the elastomer film. Since the elastomer conserves volume when it is deferred, the electrically-induced stress in the film thickness direction is converted to stress in the direction of actuation. The corrugation depth-to-period ratio is optimized in order to obtain elongation of about 33% before the metal electrode breaks.
An electrode for a bending-electrostrictive polyurethane actuator was described in Watanabe, M. et al., J. Appl. Phys. 92, 4631 (2002). The wrinkled electrode was prepared by in situ deposition of polypyrrole onto a polyurethane elastomer film that was being uniaxially drawn. After the deposition, the film was released from the drawing to make the electrode wrinkle. The bending actuator of the polyurethane film with the wrinkled electrode was improved compared to an unwrinkled one. Polypyrrole is an organic conductor, with an electrical conductivity much lower than that of typical interconnect metals, e.g., gold or aluminum. Accordingly, organic conductors have a greatly restricted applicability compared to metallic conductors.
It is desirable to provide an improved stretchable and elastic electrical interconnect of thin metal films which can be used to provide electrical connection in applications such as thin-film electronics and conformable integrated circuits.